‘And he was transfigured before them, and his clothes became glistering, extremely white, in such a way that no launderer on earth can whiten them.'
A remarkable transformation of Jesus is described in terms which show that it was really indescribable. It was a vision of the glory of heaven and of absolute purity. They ‘beheld His glory' (John 1:14) and were ‘eyewitnesses of His majesty' (2 Peter 1:16). We are probably intended to see in this a preview of ‘the glory of His Father' which would be revealed at His coming (Mark 8:38)?
‘Transfigured.' The word indicates transformation (Romans 12:2; 2 Corinthians 3:18), a change in form. The description following is thus an attempt to portray the unportrayable, a revelation of heavenly glory and purity. Words had to be found to portray something that was totally unearthly. (Luke avoids the word, possibly because among the Gentiles, without a background of Jewish apocalyptic literature, it had associations with pagan mythology and magic). The point here is that the earthly Jesus was revealed in His usually hidden heavenly glory which transformed His mortal body.
‘Glistering.' That is, shining and radiant. The word is used in LXX of the glittering of metals, especially as connected with supernatural events (Ezekiel 40:3; Daniel 10:6; also Ezekiel 21:10; Ezekiel 21:15; Ezekiel 21:28;). The idea here may be of the glory of the heavens. Matthew and Luke expand on it. Matthew adds ‘His face did shine as the sun' (Matthew 17:2 compare Revelation 1:16; Revelation 10:1 also see Matthew 28:3; Daniel 10:6 where the picture is of lightning) while Luke says ‘the fashion of His countenance was altered' (Mark 9:29). The clear idea is of One Who was of heaven and not of earth.
‘Extremely white.' Exceedingly white, whiter than white, a white beyond imagination. It was a vision of total righteousness and purity (compare Daniel 7:9 of the Ancient of Days). We are reminded by it of Him Who sits in eternity, Whose name is Holy (Isaiah 57:15). And those who are purified by God will become so glistening white (Psalms 51:7; Isaiah 1:18), and in the end will be made like Him (1 John 3:1-2). White clothes are regularly the indication of a heavenly visitant (Mark 16:5; Matthew 28:3; John 20:12; Acts 1:10; Daniel 7:9; compare Revelation 4:4; Revelation 15:6).
So Jesus is portrayed as the glorious Son of Man (Mark 8:38), and later as the even more glorious ‘only beloved Son of God' (see Mark 9:7).